Abstract:In recent decades, eutrophication, harmful algal blooms, and seasonal hypoxia in the bottom water have been frequently reported in Chinese coastal waters owing to excessive human-induced nutrient input. Phosphorus (P) is an essential biogenic element for marine phytoplankton and is important in eutrophication and harmful algal blooms of the estuarine and marine ecosystems. Sediment has a buffering effect on the P concentration in the overlying water and is an important P source for sustaining pelagic primary production. In addition, the cycling and release of P in sediments play a notable role in maintaining the water trophic status. The biogeochemical cycle of P in sediments is a key topic in marine science worldwide. Understanding the adsorption and desorption behaviors of P in sediments is necessary to comprehend P cycling and assess its potential release risk in estuarine and coastal environments. Maowei Sea is a typical tropical bay with high density oyster aquaculture where industrialization and urbanization have synchronously altered the natural ecosystem structure and marine ecological environments in northern Beibu Gulf. It is part of the Silk Road Economic Belt and the 21st-Century Maritime Silk Road, which have become an important part of the national developmental strategy of China. Human activities have affected the Maowei Sea’s ecological environment, particularly in the main estuaries of the Maowei Sea. Consequently, the average N/P molar ratios are much higher than the Redfield ratio of 16 : 1. P has become the limiting element for phytoplankton growth in the study area. The adsorption and desorption of P in sediments play an important role in the dynamic cycling of P in aquatic ecosystems. However, compared to other coastal and estuarine regions worldwide, geochemical information about P adsorption behavior characteristics in surface and core sediments in the main estuaries of the Maowei Sea has been largely ignored. The sediment acts as the “sink” or “source” of P in water through the behaviors of P adsorption/desorption, which has a significant impact on marine primary productivity and water eutrophication. However, the characteristics of the P adsorption behavior of core sediments in the estuaries of the subtropical bay remain unclear. The core sediments of the two main estuarine regions in the Maowei Sea were analyzed to examine the effects of different sediment particle sizes and salinity on sedimentary P adsorption behavior through adsorption kinetics and isothermal adsorption experiments. P speciation in the sediments before and after adsorption experiments was quantified using the improved sequential extraction (SEDEX) method, and the P adsorption mechanisms in sediments were explored. The results showed that the adsorption kinetics of P in sediments could be described by a fast and slow two-stage first-order kinetic equation, and the adsorption isotherms fitted the modified Langmuir-crossover model. The adsorption capacity of P in the sediments at different depths of the same station was relatively different, and the desorption behavior existed on both sites when the initial P concentration was low. Sediments with smaller particle sizes had a high adsorption capacity for P. The increase in salinity reduced the adsorption capacity of P in sediments, indicating that low salinity facilitated P adsorption in sediments. Exchangeable P (Ex-P) and iron-bound P (Fe-P) contents increased significantly in sediments after adsorption. The adsorption processes of P in sediments included physical and chemical adsorptions, with physical adsorption being the main process. The results may provide valuable information for further research on the P biogeochemical cycle and ecological effect, as well as contribute to the development of the beautiful bay construction and sustainable growth of the marine economy.
